Imagine that you're the skipper of a WWII submarine. Let's say it's the USS
Cod, a submarine that saw service from 1943 to 1945, and you're on the
prowl for enemy ships. What is it that you're supposed to do? How do you go
about finding and firing at a target?

The first thing you need to do is situate your boat underwater at the right
depth. For the USS Cod, you'd level off at the submarine's periscope
depth, which is 67 feet. You'd then use your search periscope to scan the
horizon. The search periscope's wide field of view allows you to see a wide
area all at once.

Then you see it. An enemy destroyer. You switch to the attack periscope. The
attack periscope is much smaller—its neck, the part that slices through
water, is only 1.4 inches in diameter, so it leaves hardly any wake as the boat
moves along.

Next up is determining the target's bearing, course, range, and speed. To get
the bearing, you center the scope's crosshairs on the ship. You call out
"mark!"; the executive officer reads out the position off a gauge on the
periscope. You determine the ship's course by guessing its "angle on the
bow." For example, if the ship is heading directly toward you, the angle on the
bow is 0 degrees. If its left side is facing you, the angle on the bow is "90
degrees port."

View through attack periscope.

There are several ways to determine range to the target. One is by
radar, but your radar antenna is under water. Another is by sonar, but its
ping could alert the enemy to your presence. Still another is your
stadiometer, a device incorporated in the attack periscope. The stadiometer
works like a sextant, but instead of giving you the angle between you and the
sun (or some other object) and the horizon, you use it to determine the angle
between you and the top of the ship's mast and the ship's waterline. Rotating a
knob on the periscope causes another image of the ship to appear in the
viewfinder. You turn the knob until the waterline of one image touches the
masthead of the other image.

Even though the sonar isn't sending out pings, your sonar operator is busy.
He's hunched over his equipment, listening to the enemy ship. With stopwatch in
hand, he determines the rate at which the enemy ship's props turn. "One hundred
and twenty five per minute," he announces. This tells you that its speed
is 9 knots (about 10mph).

The navigation officer has been busy, too. He's been listening to you and the
crew and setting the dials of the TDC—the Torpedo Data Computer. The TDC is
a strictly mechanical computer that relies on dials and gears. There are two
sections to the TDC. One is the position keeper, which keeps track of the
target, in real time, relative to your position. The navigation operator has
entered the target's bearing, course, range, and speed. The TDC also keeps
track of your position—it automatically enters in the submarine's speed and
heading and makes adjustments whenever you change speed or heading.

The other section of the TDC is the angle solver. The angle solver looks at
your position and the position of the target and calculates the course the
torpedo must take to strike the target. This would be a simple calculation if
the submarine were pointed directly at the target, if the path of the torpedo
were a straight line, and if the target were stationary. But the submarine
doesn't need to be pointed at the target because the torpedo has a gyroscope,
and the gyroscope controls the direction in which the torpedo travels. So, the
angle solver calculates the angle, or heading, that the torpedo would need to
take to reach that strike point, taking into consideration the torpedo's speed,
its reach (the torpedo does travel in a straight line after leaving its tube
for a set distance; this is called its reach), and the curved path that happens
between the end of the reach and when the gyroscope tells the torpedo that it's
on course.

You could fire one or more torpedoes now—the gyros of each of the torpedoes
in your 10 torpedo tubes are aimed at the target—but you know that if you're
patient, you'll have a better chance of making a strike because you'll be able
to fine tune the TDC settings. You lower the periscope. You wait15
minutes. You raise the periscope again. You compare the target's calculated
position to its actual position. How far off is the TDC? You make adjustments,
then lower the periscope again. More waiting. One more comparison, you think,
between the predicted and actual position should do it.

You raise the periscope. The TDC is right on. Now you're ready to fire! But
you don't give the command, as this is just an exercise.